The so-called industrial polyamide fibers are used, among other things, for netting and ropes, conveyor belt cloth, industrial machinery felts, filters, fishing lines, industrial cloth, and anchoring wire as well as brushes. As aliphatic polyamides generally have good resistance to chemicals, they are eminently suitable for paper machinery webs. In addition to generally good mechanical properties such as high tensile strength, high bending strength and abrasion resistance are required of materials which are subject to bending. These properties are highly dependent on the molar mass of the polymer. The higher the degree of polymerization of the polymer, the more stable the fibers are to bending stress.
According to the prior art, to enable polyamide fibers having high molar masses to be produced, the polyamide granulate is subjected to solid phase condensation before being spun to fibers, as described, for example, in U.S. Pat. No. 3,420,804 or in EP-PS 98 616. A disadvantage of this procedure is that the high molecular weight spinning granulate has a very high melt viscosity and can therefore be spun only poorly owing to a high build-up of pressure upstream of the spinneret. Furthermore, an uncontrolled reduction of molar mass occurs in the melt of high molecular weight granulate during the spinning process.
CH-PS 359 286 describes a process for producing high molecular weight polyamide granulate by solid phase condensation in two steps. The solid phase condensation catalysts are incorporated into the melt of the polyamide starting material and the plastics parts obtained by injection molding or extrusion are then solid phased condensed. This mode of operation is unsuitable for the production of high molecular weight polyamide fibers as the catalysts incorporated trigger uncontrolled solid phase condensation in the hot polyamide spinning melt.
Japanese 27 719/76 describes the solid phase condensation of polyamide molded shapes immersed in catalyst solution to increase the service life of highly stressed shaped articles by converting the two-dimensional molecular structure into a three-dimensional one; in other words, the polyamide is crosslinked at its surface. However, crosslinked fibers in the surface layer possess marked disadvantages in coloration and resistance to failure under repeated bending stress. In contrast to the abstract, this reference does not mention fibers but shaped articles, such as a ring traveller and sash roller.